Viscosity can be thought of as the stickiness of a fluid, or the fluids friction. Viscosity will cause the fluid’s flow to be slower as it nears the wall, as can be seen in the figure below.
Different fluids have different viscosities. Some are more viscous then others, which means that they don’t flow as easily. Also, a fluid viscosity will change as temperature changes. This is why engine oil has two numbers such as 5W-30. 5W represents the viscosity when the oil is cold, and 30 represents the viscosity when the oil is at operating temperature. For climates that have cold winters it is sometimes recommended to put a different W number in the car due to the colder climate.